The present invention relates to laser systems and, more particularly, to a probe for delivering 10.6 micron wavelength radiation and a method for such delivery.
CO.sub.2 (carbon dioxide) laser radiation is of major interest to the medical community because of the ability of the principle component of its output radiation, 10.6 microns in wavelength, to cut and/or ablate both normal and abnormal mammalian tissue when delivered at appropriate power levels. However, delivery systems for conveying the same to a mammalian tissue site have been less than ideal--this is particularly true for internal delivery. As pointed out in assignee's co-pending patent application Ser. No. 07/181,448 filed Mar. 4, 1988 entitled "DELIVERY ARRANGEMENT FOR A LASER MEDICAL SYSTEM" and naming the applicant hereof as an inventor, the delivery system must be both flexible and preferably sufficiently small at its distal end for cavity insertion and intracavity manipulation. A critical component in such delivery is the optical waveguide which directs the radiation from a source of the same to adjacent the desired mammalian tissue site. (If the waveguide is connected directly to a CO.sub.2 laser to receive its output, the source is the laser itself, whereas if the delivery system is a multi-component system such as described in co-pending application Ser. No. 07/181,448, the source can be another component of the system.)
The characteristics that are desired in a medical probe for CO.sub.2 laser radiation are known. It is desirable that it be small as mentioned above, i.e., small in dimension in the direction normal to the direction of guiding. Surgeons prefer that the greatest dimension in such direction of any part which may be inserted internally into a body, be no greater than 1 millimeter. It is also quite important that it be flexible, i.e., capable of being flexed, without major variations in its transmission capability. In connection with the latter, a surgeon has to be able to expect basically the same output power from the probe irrespective of variations in probe flexing. Moreover its is important that the portion of the guide, the operative part of the probe, which is to be inserted into a body be fairly long, e.g., about 75 centimeters in length. This means that the transmission loss per unit of length has to be minimized.
Much effort and investigation has been undertaken in an effort to provide an appropriate medical probe. In this connection, efforts have been made to extend the wavelength range of dielectric optical fibers of the type used with other wavelengths formed by extruding crystals or drawing special glasses. For example, reference is made to the paper entitled "Hollow-Core Oxide-Glass Cladding Optical Fibers for Middle Infrared Region" (July 1981) by T. Hidaka, et al., published in THE JOURNAL OF APPLIED PHYSICS, Vol. 52, No. 7, page 4467 et seq. These efforts, though, have not been satisfactory since such materials have transmission loss for CO.sub.2 radiation. Such transmission loss is particularly significant when the materials form bent structures, such as curved probes, since much of the input radiation is absorbed. The paper entitled "Fiber Optic Trends" authored by the instant inventor and appearing on page 51 in the July 1987 issue of PHOTONICS SPECTRA provides a basic overview of the developments with respect to IR fibers (optical waveguides for CO.sub.2 radiation). As brought out in such paper, CO.sub.2 radiation transmission in most of such fibers is significantly decreased when the fiber is flexed to a small bend radius An investigation with respect to use of a particular metal probe which is dielectrically coated is described in the paper entitled "A Flexible CO.sub.2 Laser Fiber for Operative Laparoscopy" (July 1986) by Baggish, et al. appearing in FERTILITY AND STERILITY, volume 46, page 16. While it is stated that such metal IR fiber is "flexible," the effect of flexing on transmission is not discussed. It is expected to be great based on other metal waveguides, though, since a metal is quite lossy and differential flexing will result in significantly great differential absorption. Moreover, the lossy nature of such a guide will limit its overall length.
There is an anomalous dispersion phenomenon that is associated with transmission of 10.6 micron radiation through certain non-metallic, hollow waveguides. As pointed out in the paper entitled "Dispersion Phenomena in Hollow Alumina Waveguides" (1985) authored by Jenkins, et al. and appearing at pages 1722 et seq. in the IEEE Journal of Quantum Electronics, Vol. QE-21, it has been discovered that alumina waveguides have an abrupt change in attenuation which is directly related to the wavelength of the radiation which is transmitted. It has been found that the attenuation of 10.6 micron radiation by a hollow air core alumina waveguide is significantly lower than it is for other longer or shorter wavelengths, such as 9.6 micron radiation. This phenomenon has been associated with the index of refraction (n) of the alumina relative to the wavelength. That is, this relatively unexpected phenomenon has been associated with the index of refraction of the alumina becoming less than that of the air core over a short range of wavelengths.
Most polycrystalline materials satisfactory for n&lt;1 hollow guides are too brittle to provide flexing. Probe constructions using polycrystalline material as a cladding in which some flexing can be achieved have been considered. For example, it has been considered to provide a thin coating or layer of a polycrystalline material within an otherwise flexible substrate tubing. While such a construction may permit flexing, relatively high transmission losses would be expected due to the polycrystalline nature of the coating.
Although there has been much investigation and effort to arrive at a satisfactory probe for delivering CO.sub.2 radiation, to date these efforts have not produced an entirely satisfactory probe or method of delivering 10.6 micron radiation to a mammalian tissue site.